The present invention relates to a video signal recording apparatus suitable for recording a video signal, especially a video signal representing a still image.
Recently, video signal recording apparatuses have been commercially available in place of cameras using silver chloride films to record a video signal representing a still image. The video signal specifications resemble those of current television systems. For this reason, a still image obtained by reproducing a recorded video signal is inferior in resolution, hue, etc. to a picture taken by a conventional camera. Therefore, there has arisen a demand for improved image quality in such video signal recording apparatuses for still images.
The most significant problem in the improvement of image quality is the fact that a frequency bandwidth of the video signal must be shifted to a higher bandwidth. The specifications of the high quality television system proposed by NHK (Nippon Hosso Kyokai) are given as follows. Values given in parentheses are specifications of the current NTSC television system.
Luminance (Y) signal bandwidth: 20 MHz (4.5 MHz) PA0 Chrominance signal PA0 Chrominance subcarrier frequency: 24.3 MHz (3.58 MHz) PA0 Horizontal Scanning frequency: 33.75 kHz (15.74 kHz) PA0 Number of frames per second: 30 (30) PA0 Number of horizontal scanning lines: 1125 (525).
Wide band chrominance signal (C.sub.W): 7 MHz (1.5 MHz) PA2 Narrow band chrominance signal (C.sub.N): 5.5 MHz (0.5 MHz)
In the high quality television system, the frequency bandwidth falls in the range of 20 MHz to 30 MHz. The frequency bandwidth is increased to several times that of the current NTSC system.
A conventional video signal recording/reproduction system of this high quality television system is illustrated in FIG. 1. R, G and B color component signals are supplied from a high quality TV camera 10 having three color tubes to a matrix circuit 12, thereby obtaining a luminance signal Y, a wide band chrominance signal C.sub.W and a narrow band chrominance signal C.sub.N. A sync signal is superimposed on the luminance signal Y by a sync mixer 14 and the resultant signal is then supplied to a first channel magnetic head 20 through a frequency modulator 16 and a recording amplifier 18. The signals C.sub.W and C.sub.N are sequentially supplied to a line sequence converter 22 and are combined. A composite signal from the line sequence converter 22 is supplied to a second channel magnetic head 28 through a frequency modulator 24 and a recording amplifier 26. A carrier frequency of the frequency modulators 16 and 24 is set to be 34.5 MHz. The magnetic heads 20 and 28 record signals on a magnetic sheet 30 having a diameter of 0.5 m. The magnetic sheet 30 is rotated by a motor 32 at a rotational frequency of 60 rps and moves at a speed of 84 m/s relative to the magnetic heads 20 and 28. An output luminance signal from the sync mixer 14 is supplied to a sync separator 34, so that the sync signal can be separated from the luminance signal. A clock generator 36 generates a clock signal having a frequency of 60 Hz in response to an output from the sync separator 34. The clock signal is then supplied to the motor 32 through a drive amplifier 38. Therefore, the motor 32 causes the magnetic sheet 30 to rotate at the rotational frequency of 60 rps.
Switches 40 and 42 are arranged between the recording amplifier 18 and the magnetic head 20 and between the recording amplifier 26 and the magnetic head 28, respectively, so that signals from the magnetic heads 20 and 28 may be supplied to a reproduction system. The signal from the first channel magnetic head 20 is applied through the switch 40 to a reproduction amplifier 44. The output of the amplifier 44 is applied to an FM demodulator (DEMOD) 48 through an equalizer 46. The output of the demodulator (DEMOD) 48 is used as the luminance signal Y. The signal from the second channel magnetic head 28 is applied through the switch 42 to a reproduction amplifier 50. The output of the amplifier 50 is applied to a line sequence converter 54 through an FM demodulator 52, to produce a wide band chrominance signal C.sub.W and a narrow band chrominance signal C.sub.N. The luminance signal Y, the wide band chrominance signal C.sub.W and the narrow band chrominance signal C.sub.N are supplied to a matrix circuit 6. The R, G and B color component signals are then supplied from the matrix circuit 56 to a high quality CRT monitor 58. As a result, a color still image can be reproduced by 1125 horizontal scanning lines. The monitoring reproduction system is operated in the recording mode such that the outputs (indicated by broken lines) from the frequency modulators 16 and 24 are directly supplied to the FM demodulators 48 and 52, respectively.
The high quality video recording/reproduction apparatus of this type has a large size and is costly, since a large magnetic sheet is used to record/reproduce the video signals. Therefore, the apparatus of this type must comprise a fixed installation, and cannot be used in place of a conventional camera. A portable video signal recording/reproducing apparatus must be compact, lightweight, and less costly, and have a low power consumption (battery operated). The apparatus shown in FIG. 1 cannot be arranged as a portable apparatus since a high frequency band signal is directly recorded in a large-capacity memory (magnetic sheet) without performing frequency band conversion through a buffer memory such as a frame memory.
FIG. 2 shows a conventional system for recording one-frame video signals in a frame memory, these signals being produced by a TV camera used in the current NTSC system. Analog color component signals W (white), Ye (yellow), Cy (cyan) and G (green) obtained by photoelectric conversion in units of pixels, in a solid-state image pickup device 60 having a CCD, for example, are supplied to a matrix circuit 64 through a pre-amplifier 62. R, G and B color component signals are supplied from the matrix circuit 64 to a process IC 70 through an FPN suppression IC 66 and a low-pass filter 68. The outputs from the FPN suppression IC 66 are also supplied to a V smear compensator 72. Outputs from the process IC, 70 are supplied to an encoder IC 74 and are superimposed with a sync signal from a sync IC 76, thereby obtaining an NTSC video signal. The NTSC video signal from the encoder IC 74 is digitally written in a frame memory 82 through a buffer 78 and an A/D converter 80. The written frame image can be checked by an electronic view finder 84 connected to the output terminal of the buffer 78.
In the NTSC system, the frequency bandwidth of the video signal falls within the range (industrial) 0 to 4.5 MHz or the range (commercial) 0 to 2 MHz. In order to A/D-convert a video signal, the sampling frequency is set to three to four times a chrominance subcarrier frequency f.sub.SC. Since the subcarrier frequency f.sub.SC is 3.58 MHz in the NTSC system, the sampling frequency, which is equal to four times the NTSC subcarrier frequency, is 14.32 MHz. When one-sample data of one pixel is A/D-converted to 8-bit data, the frame memory for storing one-frame video signals must have a capacity of 3.8 Mbits (.apprxeq.8.times.14.32.times.1/30).
If the video signal bandwidth input to the A/D converter 80 falls within the range 0 to 10 MHz, the Nyquist rate is set to 14.32 MHz, so that beat noise occurs between the video signal having a frequency close to the Nyquist rate and the sampling signal of the A/D converter 80. For this reason, the frequency band of the video signal is limited by an LPF 68, as indicated by the dotted line in FIG. 3.
In the system of the type described above, the signals from the pixels of the solid-state image pickup device 60 are converted to a single analog NTSC signal, and the converted signal is written in the frame memory 82 after A/D-conversion. For this reason, the data from each pixel may not be written per se in the frame memory due to a slight deviation in sampling frequency of A/D conversion and to a phase deviation of the analog circuit.
When the recording system shown in FIG. 2 is applied to the high quality image as previously described, the sampling frequency of the A/D converter becomes 97.2 MHz (=24.3.times.4). As a result, given current semiconductor techniques, a portable apparatus is not possible when cost, size and power consumption are considered. Furthermore, the frame memory must have a capacity of 26 Mbits (.apprxeq.8.times.97.2.times.1/30), which precludes production of a portable video signal recording/reproduction apparatus.